The British Government has accused Iran of attempting to crush free speech after reports that a detained blogger was tortured to death in prison.

Anti-government activists in Iran say Sattar Beheshti was arrested by police on October 30 after posting messages on Facebook calling for freedom of speech and human rights.

Authorities are said to have informed his family that he had died on Tuesday.

British Foreign Minister Alistair Burt has said he was shocked at the reports.

"Beheshti's only crime appears to be advocating the defence of human rights on the internet," he said.

"Tragically, we have seen many similar cases of Iranians being locked up and mistreated in prison for expressing such views.

"If these reports are true, this is yet another disgraceful attempt by the Iranian government to crush any form of free expression by its citizens," the minister added, calling on authorities urgently to confirm what has happened to the blogger.

The 35-year-old was from the city of Robat Karim, around 30 miles (48km) south-west of the capital Tehran.

The Iranian news website Kaleme, which has links to opposition leader Mir Hussein Mousavi, says Beheshti's family was contacted two days ago by authorities to inform them of his death.

Protests in Iran have flared up again

In an interview with the website, Beheshti's sister said: "They told [my brother] to buy a grave, and to collect the body ... That’s it!

"We know nothing else. We don’t know why they killed him, or what exactly happened. We don’t know what happened. My brother was well when he left the house. He left on his own two feet. Everyone saw that he was healthy. My brother didn’t even take headache pills."

A Facebook tribute page has already been set up in Beheshti's name, with numerous postings expressing anger at his reported death.

One user issued a rebuke to the Iranian authorities, saying: "The fear in our hearts is no longer a fear... If your motto is 'we will arrest you and torture you,' our motto is 'you cannot silence us.'"

Another said simply: "How can we be silent?"

Beheshti is believed to have been arrested by the Iranian police cyber unit, which was reported to have been set up in response to the major anti-government protests in 2009.

The so-called 'Green Revolution' was notable for the way activists harnessed the power of social media such as Facebook and Twitter to spread their message, but a fierce crackdown by authorities eventually brought an end to the protests.

Recently there has a been a resurgence in vocal anti-government sentiment in Iran, driven in large part by the biting economic impact of international sanctions imposed as a result of Iran's alleged pursuit of a nuclear weapons programme.

Last month, Iranian police used teargas to disperse hundreds of protesters in Tehran who were demonstrating against President Mahmoud Ahmadinejad's handling of the economy and currency crisis, which has seen the value of the Rial plunge on international markets.